Coccidioidomycosis, more commonly known as Valley Fever, is a fungal infection caused by inhaling either Coccidioides immitis or Coccidioides posadasii conidia that are found in the soil in arid to semi-arid regions of the Americas. Both Coccidioides species, Coccidioides immitis and Coccidioides posadasii, share an asexual life cycle characterized by two stages, the saprobic cycle and the parasitic cycle. The saprobic cycle is found in the environment. During the saprobic cycle, these fungi alternate between two main cell types in the environment: arthroconidia and hyphae. The arthroconidial phase is believed to be the major infectious particle found in the environment. When a susceptible host inhales an arthroconidium, the parasitic cycle begins. The parasitic life cycle is initiated when arthroconidia enlarge and transform into immature spherules. Spherules undergo free nuclear division and begin developing endospores. During an active parasitic cycle, an arthroconidium can transition to a mature rupturing spherule within five days after the initial exposure (see Reference 1). Once the spherule ruptures and releases endospores, each endospore can develop into a new spherule and the parasitic cycle can continue growing exponentially.
It is estimated that 40% of human Coccidioides infections are symptomatic (see Reference 2). Acute or chronic pulmonary disease is most common manifestation, however disseminated disease occurs in approximately 1% of Coccidioides infections (see Reference 2). Certain factors such as African or Filipino ancestry, immunosuppression, pregnancy, and male gender increase the risk of a disseminated infection (see References 2 and 3). The remaining 60% of human Coccidioides infections are asymptomatic and generally result in clearance or development of asymptomatic lung nodules. Morphological variation is highest during early days of infection, when the inhaled environmental conidia are switching to the parasitic lifestyle.